Calculating attic ventilation

All too often an existing home has far too little attic ventilation and some difficult choices for adding more. We all know the existing, although somewhat old, method of one ft² of net free vent area (NFA) for every 300 ft² of attic floor space and double that if the ceiling plane has not been well air sealed. Whatever that number comes out to be should be roughly divided half high and half low. 60/40 and 40/60 cold country vs ac country are options. But when you are short on vent area and cash, here is a detail not often mentioned.

The NFA is only part of the ventilation equation. The other half is the pressure across those vents. A 10 ft attic will have twice the attic stack pressure of a 5 ft attic. Now, I would have to do a bunch of reading to find out what height attic those old calculations were based upon, but a simple fudge factor would be, short attics need more and tall attics may get by with less.

Feel free to post any references that show how to adjust for attic height, fudge is good but real numbers taste better.

Replies to This Discussion

<We use it because we are stuck with it!> I agree that's what we are doing, but that doesn't make it right.

<No one likes change!> As you know, I've already encountered that! But it would be hypocritical of us to complain about other contractors following old habits while we have in front of us a standard from the same "hand me down" origin.

<No one really understands why attic ventilation needs to happen.> Although home owners may not understand, we (all auditors and related contractors) should.

<How to make it work correctly … with incorrect work... > That is often the challenge with a retrofit and add to that, the solution will often have a very limited budget. Thus it is good to know what is needed in each situation and whether there exists any wiggle room.

<It has gotten entrenched into code and we are stuck.> Stuck is not stopped, it just means we are going to have to dig a little to get out, and get a little muddy in the process.

There are few problems that a bunch of money can't fix, but very few owners will pop for those expensive solutions. If a house needs more ventilation, the owners deserve to know if the estimated solution is a wag or one based upon their home and their location. The 1:300 is a wag and we can do better. I was hoping someone had.

Bud, I know you believe every house is a unique structure and worth individual attention.

If you have a 900 sf ranch with a full basement, or 1800 sf conditioned space. ca 1965. No vaults etc. The 1:300 is 3 sf of attic ventilation. That would be NFA. Ridge Vent is 11 NFA inches per foot. Turtles are rated at 50 NFA for a 10 inch diameter (I don't buy that one, but I'm not rating them.) A 6 x 16 soffit rates out at 64 NFA.

3 sf = 432 sq in NFA. Does not take much.

The 1:300 was about 2x of the 1:576, which Rowley said was OK. It is half the 1:186 that he thought was too much. He was also talking about total ventilation - Gable to Gable. No high / low.

John, to be honest, I don't think we can consider any of the Rowely work as relevant, 1939, cross ventilation only, and all done in a lab. One could say we have had 70 years of field work to add credibility to his conclusions, however, there isn't a one size fits all solution.

If we look at just the cross ventilation limitations, as compared to what we experience with a good ridge and soffit combination, there is no way equal vent areas will produce the same effective air flow. That alone should allow one to install substantially less NFA for a ridge/soffit install.

Then, 1939 is even older than I am. A lot has changed since then and our understanding of attic issues has improved, I hope.

Your example is for 1:300, which assumes an air sealed ceiling, but that is not always the case and certainly wasn't way back then. So air sealed or not is a factor of two, height can be another factor of two and other variables can probably push that 3 ft² to anywhere between 10 ft² to zero. I'm exaggerating, but there is a cost that goes with ventilation and we need a better way to pick a number.